The invention relates to a speed change transmission including a continuously variable toroidal transmission with a planetary aggregate drive structure.
In a known variable-speed transmission arrangement (U.S. Pat. No. 6,059,685), at least one double planetary gear structure, which consists of a main planet meshing with a secondary planet, is mounted on the double-webbed planet carrier of the output transmission. The planet carrier is mounted fixedly for rotation with the input shaft via a central intermediate shaft. The toroidal transmission is a two-chamber design, and is connected, by means of its one central driving disc, to the input shaft, by means of its other central driving disc to the planet carrier and by means of its central driven discs to the concentric intermediate shaft, the latter, in turn, being connected fixedly in terms of rotation to one of two sun wheels meshing with the main planet. The secondary planet meshes with an outer ring wheel which can be drivingly connected to the output shaft by means of the first shift clutch for the lower driving mode at lower driving speeds. The other sun wheel meshing with the main planet can be drivingly connected to the output shaft by means of the second shift clutch for the upper driving mode at higher driving speeds.
This known variable-speed transmission arrangement is already designed, in terms of its transmission ratio, with what is known as a geared-neutral function, by means of which, with the first shift clutch for the lower driving mode being in an engaged state, the rotational speed of the transmission members connected fixedly in terms of rotation (ring wheel of the output transmission and output shaft) is equal to zero when the part transmission ratio of the toroidal transmission is set at a negative absolute value lower than 1 which is dependent on the respective number of teeth of the output transmission. It is consequently unnecessary, for the start-up operation, to have a special drive component, such as, for example, a start-up converter or start-up clutch.
Furthermore, in this known variable-speed transmission arrangement, a transmission ratio designated as what is known as a synchronous point can be selected, in which the transmission members connected to the first shift clutch, on the one hand, and the transmission members connected to the second shift clutch, on the other hand, each have the same rotational speed, so that a change in the torque path from one clutch to the other, and therefore the transition from one driving mode to the other, can be completed in a way which is jolt-free, that is to say not noticeable by the driver.
By virtue of the above-described transmission configuration of the known gear shift transmission arrangement, the outer sun gear of the output transmission, which outer sun gear is connected to the first shift clutch for the lower driving mode, necessarily has a direction of rotation opposite to the direction of rotation of the input shaft, so that a reversing mode with a suitable transmission ratio is possible when the first shift clutch is engaged and, in the toroidal transmission, a negative partial transmission ratio dependent on the respective number of teeth in the output transmission is set with an absolute value lower than 1.
In a known gear-change transmission arrangement of a different type (EP 0 942 199 A2) operating with a continuously variable toroidal transmission, use is made not of a planetary output transmission, but of a countershaft which is arranged eccentrically outside the toroidal transmission and parallel to the input shaft and which is connected in each case by means of an axially offset drive, on the one hand, directly to the two central driven discs of the toroidal transmission designed on the two-chamber principle, and, on the other hand, via a first shift element in the form of a range clutch for forward driving, to the output shaft coaxial to the input shaft. In order to generate the frictional connection, necessary for torque transmission, between the respective roller and the toroidal frictional surfaces of the associated driving disc or driven disc in the toroidal transmission, the one first driving disc is connected to the input shaft, to be driven via a start-up element in the form of a torque converter, by means of a torque-dependent pressure device, the rolling-body-like intermediate members of which are in engagement with inclined surfaces formed on one or both device halves. The other second driving disc is with the input shaft either in rotationally fixed, that is to say direct drive connection or, via the pressure device of the first driving disc, in indirect torque-dependent drive connection. A simple planetary reversing transmission for forming a reverse gear is arranged in the force path between the second driving disc of the toroidal transmission and the coaxial output shaft, by which reversing transmission the inner central wheel is connected to the second driving disc and the outer central wheel is connected to the output shaft, in each case fixedly in terms of rotation, and also the planet carrier is connected to a non-rotating case part by means of a second shift element in the form of a reverse-gear brake and to the outer central wheel by means of a third shift element in the form of a direct-drive clutch. The transmission control is designed in such a way that in each case only one of the three shift elements can be activated, whilst at the same time the other two necessarily remain deactivated. Three transmission states are thereby defined, of which, in a first transmission state, the entire drive power is led to the output shaft via the toroidal transmission, bypassing the reversing transmission, as a result of the activation of the range clutch for forward driving and, in a second transmission state, as a result of the activation of the direct-drive clutch, the entire drive power is transmitted, bypassing the toroidal transmission, from the input shaft to the output shaft via the reversing transmission rotating as a block, whilst, in a third transmission state, as a result of the activation of the reverse-gear clutch, the entire drive power flows, bypassing the toroidal transmission, from the input shaft to the output shaft via the reversing transmission shifted to its reverse-gear transmission. Although the toroidal transmission is non-loaded both in direct drive and in reverse gear, it is nevertheless subjected to the full drive power in the variable-ratio forward-driving range, because operation with power splitting and subdivision into at least two reduction ranges is not possible because of the absence of a pick-off transmission. Moreover, a variable-speed transmission arrangement of this other type, with a torque converter and a bulky outer countershaft, cannot readily be accommodated in a transmission tunnel of a conventional vehicle.
DE 199 11 297 A1 specifies, for a variable-speed transmission arrangement of yet another type, with a continuously variable single-flow or multi-flow friction-wheel transmission with cone-like friction wheels arranged in opposition and rolling on one another directly or via a cylinder-like hollow body, that the change in the transmission ratio should take place by the axes of the friction wheels being tilted to a greater or lesser extent in relation to one another and intersecting at different angles. For this variable-speed transmission arrangement of another type, maximum efficiency in the important operating ranges and, in particular, also in the longer gears effective for consumption is claimed, and reference is made to the fact that it is at the same time of high performance and outstandingly adjustable. These advantages are to be achieved in that the drill-rolling ratio (spin) is smaller in operating ranges important for overall consumption than in the other operating ranges, in that the cone angle of the respective rolling tracks is smaller than in the remaining rolling tracks. Additionally or alternatively to this design of the drill-rolling ratio, in this variable-speed transmission arrangement of another type the non-positive friction-wheel transmission is to be capable of being bypassed, in the gear most important for overall consumption, by means of a shiftable direct through-drive or a shiftable gearwheel stage acting directly on the differential.
In the variable-speed transmission arrangement according to the invention, the input shaft is capable of being drive-connected to the output shaft by means of a third clutch, so as to bypass the toroidal transmission, thus providing a continuously variable vehicle transmission having, along with a compact design, a high efficiency in the consumption-relevant transmission ratios, which is achieved by virtue of the direct drive transmission which can be selected, as required, by means of the additional clutch.
In the variable-speed transmission arrangement according to the invention, the direct power transmission arrangement can be provided for the higher speed driving mode. During the change of the torque path from the third clutch associated with the direct drive arrangement to the second clutch associated with the variable transmission ratios of the upper driving mode, the rotational speeds of the connected transmission members can be synchronized at the second clutch by the adaptation of the variable part transmission ratio of the toroidal transmission, so that the change takes place in a way which is jolt-free, that is to say not detectable by the driver.
In the variable-speed transmission arrangement according to the invention, the transmission control can be designed in such a way that, if the desired value to be selected lies in the range of the constant overall transmission ratio, a comparison is carried out as to whether the current engine operating point leads, in conjunction with a constant transmission ratio, to a lower fuel consumption. If this is the case, the transmission is put into direct drive.
In the variable-speed transmission arrangement according to the invention, the transmission control can be designed, furthermore, in such a way that, in the case of a high power requirement, the transmission is operated as far as possible in direct drive. This measure has an advantageous effect particularly during acceleration operations and at high driving speeds.
As a rule, a variable-speed transmission arrangement of a motor vehicle is controlled for more than 60% of the entire driving time at transmission ratios lower than 1. The influence of these transmission ratios on the useful life of, in particular, the toroidal transmission is correspondingly high. In many driving situations, particularly during long distance travel or under full load, continuously variable speed conversion is actually not required.
In the variable-speed transmission arrangement according to the invention, it is therefore beneficial that the power flow can be led past the toroidal transmission via a bypass path, so as not to wear the toroidal transmission unnecessarily. By the direct drive according to the invention being in operation, the load on the toroidal transmission can, in conjunction with an appropriately adapted transmission control, be reduced by more than 20%. As a result of the reduced load on the toroidal transmission, the latter is afforded advantages in terms of useful life, construction size and efficiency.
In toroidal transmissions, the geometry of the traction surface on the respective pivotally movable intermediate roller, which is in frictional contact in each case with a driving disc and driven disc, often represents a compromise between useful life and efficiency. High efficiency is achieved with a frictional (contact) surface which is as small as possible; conversely, the load increases since the force acts on a smaller surface. By virtue of the lower load on the toroidal transmission which is achieved in the variable-speed transmission arrangement according to the invention, the geometry can be modified in such a way that higher efficiency is achieved.
As a result of the higher efficiency achieved in the variable-speed transmission arrangement according to the invention, the maximum possible final vehicle speed can be increased, so that there is no longer the disadvantage of a lower final vehicle speed in conventional continuously variable transmission arrangements, as compared with the higher final vehicle speeds possible in the case of multi-step transmissions.
In an embodiment of the variable-speed transmission arrangement according to the invention, the constant overall transmission ratio is 1:1 and represents therefore a direct drive.
In the variable-speed transmission arrangement according to the invention, this direct drive can be implemented by the structural incorporation of the appropriate third clutch directly or indirectly between the input and the output shafts.
Even when the direct drive is implemented in the variable-speed transmission arrangement according to the invention, two tooth engagements are also included in the power transmission, but they may have an efficiency higher than 99%, so that it is still possible to achieve an overall efficiency which is well above that of the variable transmission ratios of the upper driving mode.
In the variable-speed transmission arrangement according to the invention, the constant overall transmission ratio for the upper driving mode may be designed to be unequal to 1, that is to say, for example, may have an overdrive characteristic.
However, the constant overall transmission ratio may be equal or unequal to 1.
In a particular embodiment of the variable-speed transmission arrangement, the constant overall transmission ratio is designed to be unequal to 1.
In an advantageous embodiment, the mechanical outlay necessary for the third clutch of the constant overall transmission ratio is kept low because the clutch components of the first clutch (for the lower speed driving mode) and of the third clutch are combined in a common subassembly, so that the additional construction space requirement is relatively small.
The invention will be described in more detail below in connection with eight embodiments illustrated diagrammatically in the accompanying drawings: